<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
<title>THE BOOST MPL LIBRARY: Broken Integral Constant Expressions</title>
<link rel="stylesheet" href="../style.css" type="text/css" />
</head>
<body class="docframe">
<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./portability.html" class="navigation-link">Prev</a>&nbsp;<a href="./incomplete-support-for.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group">Back&nbsp;<a href="./incomplete-support-for.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./portability.html" class="navigation-link">Up</a>&nbsp;<a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
<td class="header-group page-location"><a href="../index.html" class="navigation-link">Front Page</a> / <a href="./technical-details.html" class="navigation-link">Technical Details</a> / <a href="./portability.html" class="navigation-link">Portability</a> / <a href="./broken-integral-constant.html" class="navigation-link">Broken Integral Constant Expressions</a></td>
</tr></table><div class="header-separator"></div>
<div class="section" id="broken-integral-constant">
<h1><a class="toc-backref" href="./portability.html#id74" name="broken-integral-constant">Broken Integral Constant Expressions</a></h1>
<p>This is probably the most surprising of the portability issues
we're going to discuss, not least because for many C++ programmers, their everyday 
experience seems to indicate no problems in this area whatsoever. After all,
integer compile-time computations along the lines of:</p>
<pre class="literal-block">
enum flags {
      flag1     = (1 &lt;&lt; 0)
    , flag2     = (1 &lt;&lt; 1)
    , flag3     = (1 &lt;&lt; 2)
    ...
    };
</pre>
<p>are <em>very</em> commonplace in C++, and there is hardly a compiler out
there that cannot handle this correctly.  While arithmetic by
itself is indeed rarely problematic, when you are trying to mix it
with templates on certain deficient compilers, all kinds of new
issues arise. Fortunately, as with the rest of the portability
issues we're discussing here, the problem fades into past as new
compiler versions are released. The majority of most recent
compilers of many vendors are already free from these issues.</p>
<div class="section" id="the-problem">
<h2><a name="the-problem">The Problem</a></h2>
<p>The problem is in fact multi-faceted; there are a number of
different subissues.  Some are present in one set of compilers,
some are in another, and it's not uncommon for a code that works
for one compiler to break another one and vice-versa. If this
sounds like a maintenance nightmare to you, it is!  If you are
interested in the specific list of issues, please refer to John
Maddock's excellent &quot;<a class="reference" href="http://www.boost.org/more/int_const_guidelines.htm" target="_top">Coding Guidelines for Integral Constant
Expressions</a>&quot; summary.  For the purpose of our discission here, it
is sufficient to say that if your code has to work on one of the
compilers listed as problematic in this area, you can safely assume 
that if you decide to fight them on a case-by-case basis, chances 
are that you won't be able to maintain your sanity for very long.</p>
</div>
<div class="section" id="the-symptoms">
<h2><a name="the-symptoms">The Symptoms</a></h2>
<p>On the positive side, when you have an issue with integral
arithmetic, the diagnostics are almost always straightforward:
usually the error message refers you to the exact place in the code
that is causing problems, and the essence of issue is obvious from
the error's text, or it becomes obvious once you've encountered the
same error a couple of times.  For instance, if we throw this
well-formed fragment at MSVC 7.1 (otherwise an excellent compiler!)</p>
<pre class="literal-block">
void value();

// compares two Integral Constants
template&lt; typename N1, typename N2 &gt; struct less
    : bool_&lt; (N1::value &lt; N2::value) &gt; // line #8
{
};
</pre>
<p>we get:</p>
<pre class="literal-block">
portability.cpp(8) : warning C4346: 'N2::value' : dependent name is not a type
        prefix with 'typename' to indicate a type
        portability.cpp(10) : see reference to class template instantiation 'less&lt;N1,N2&gt;' being compiled
portability.cpp(8) : error C2143: syntax error : missing ',' before ')'
portability.cpp(9) : error C2143: syntax error : missing ';' before '{'
portability.cpp(10) : error C2143: syntax error : missing ';' before '}'
portability.cpp(10) : fatal error C1004: unexpected end of file found
</pre>
<p>The errors themselves are far from being ideal, but at least we are
pointed at the correct line and even the correct part of the
line. The above basically reads as &quot;something's wrong between the
parentheses&quot;, and that plus the &quot;syntax error&quot; part is usually
enough of the clue.</p>
</div>
<div class="section" id="the-solution">
<h2><a name="the-solution">The Solution</a></h2>
<p>Despite the fact the problems are so numerous and multi-faceted and
the workarounds are conflicting, the problems can be hidden
reliably beneath a library abstraction layer.  The underlaying idea
is very simple: we can always wrap the constants in types and pass
those around. Then all that is left is to implement algebraic
metafunctions that operate on such wrappers, once, and we are home
safe.</p>
<p>If this sounds familiar to you, probably it's because you have
already took a look at the MPL and know that the approach we just
described is in fact <em>the</em> standard way of doing arithmetic in the
library.  Although it's motivated by more general observations,
this fact comes very handy for the library users that care about
portability of their numerically-heavy metaprograms.  The MPL
primitives are already there, and more importantly, they already
implement the necessary workarounds, so your numeric code just
works.  In fact, if you stay within the library's type-wrapper
idioms, these particular problems never &quot;leak&quot; out of its
abstraction layer.</p>
<p>On a final note, there is a price of avoiding built-in arithmetics
altogether, namely decreased readability and, on some compilers,
increased compile-time overhead. Still, in majority of cases, the
benefits of type-based arithmetics overweight its small
shortcomings.</p>
</div>
</div>

<div class="footer-separator"></div>
<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./portability.html" class="navigation-link">Prev</a>&nbsp;<a href="./incomplete-support-for.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group">Back&nbsp;<a href="./incomplete-support-for.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./portability.html" class="navigation-link">Up</a>&nbsp;<a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
</tr></table></body>
</html>
